Customizing specific system email templates

Hi there,

I want to change the subject line of weekly digest emails.

As far as I understand the automatic digest/summary mails are treated differently and the template can not be edited as described here.
I found this post Customize subject format for digest emails but its very old and the solution doesn’t exist anymore.

Why are digest/summary emails treated differently?

Where can I define the subject line for those?

Thanks a lot for any help!